Thor could use his knowledge of Asgardian medicine or magic to see if he can help. He might be a bit gruff on the outside but would be concerned. Black Widow would use her espionage skills to quickly assess the situation and look for any signs of foul play. She'd also be very protective of Peter, seeing him as a young and vulnerable member of the team.
Tony Stark might immediately rush to his side and start scanning him with his high - tech gadgets to find out what's wrong. He'd be worried as he sees Peter as a sort of protege. Captain America would probably be more of a calming presence, trying to keep everyone else from panicking while waiting for medical help if needed.
The other Avengers would likely be very concerned. They might immediately stop whatever they are doing to tend to him. For example, Tony Stark could use his technology to try and figure out what's wrong with Peter. Captain America would probably be the one to keep everyone calm and organized while they figure out a plan to help Peter.
In the fanfic, if Peter's future involves him becoming a more powerful superhero, Thor might be excited to see another strong ally. He could offer to train Peter in combat. Bruce, on the other hand, would be concerned about how Peter's powers could affect his mental state. He might offer to help Peter understand the scientific aspects of his powers. Clint would probably be the one cracking jokes, but also secretly be in awe of Peter's growth and potential.
Peter usually blushes a lot. Since he's a shy and somewhat awkward teenager, his face turns bright red when the Avengers embarrass him. He might stutter while trying to defend himself or come up with an excuse.
The Avengers would initially be in shock when they find out Peter is sick. Clint Barton would be the one to keep a calm head and start organizing things. Wanda Maximoff would try to use her powers to ease Peter's pain if possible. Vision would use his vast knowledge to try and find a solution. They would all be very concerned and do everything they can to make Peter better, from getting the best medical help to providing a safe and comfortable environment for him to recover.
Peter might be very defensive at first. He could argue back, trying to explain his actions. After all, he is a teenager with his own ideas and he may think that the Avengers don't fully understand his perspective.
His Aunt May would be extremely worried and would probably fuss over him, making sure he takes his medicine and rests. Ned would be constantly by his side, bringing him things from school or just keeping him company. MJ might be a bit more stoic on the outside but would also show her concern in her own way, like bringing him books to read while he recovers.
One possible reason could be overexertion. Peter Parker is often depicted as taking on a lot of responsibilities both as Spider - Man and as an Avenger. He might be pushing himself too hard in a fight or during a mission, leading to a collapse. Another reason could be some sort of external attack that affects his physical or mental state, like a new type of villain's weapon that drains his energy.
He might also get angry. Peter has a strong sense of justice. If he feels that the Avengers are being unfair, he could stand up to them. He'll use his quick wit and his powers to show that he's not going to be pushed around easily.
Iron Man, on the other hand, might be more direct. He could use his technology to track Peter's actions and confront him. He might threaten to expose Peter's identity if he doesn't stop. And Thor would probably be angry at first. He values his fans a great deal and seeing Peter bully them would make him want to use his hammer to teach Peter a lesson, but then he would also try to understand the situation.
Some Avengers might be initially skeptical. Thor, for example, might not understand the power of a 'spider - boy' at first. But others like Captain America would probably see his potential and welcome him. Black Widow might be intrigued by his skills and try to train him further in espionage - related aspects.